1. Field of the Invention
The invention relates to a method for the operation of a stirring ball mill having a preferably cylindrical, rotatable barrel with an inlet at its one end and an outlet at its other end, in which a stirring shaft provided with stirring means is rotatable, and to ball mills with stirring mechanism especially for this method.
2. Description of the Related Art
U.S. Pat. No. 2,592,994 disclosed back in 1952 a stirring ball mill in which the rotor--or also the barrel of the mill--rotates at such a speed that the entire bed of the mixture being ground is forced against the inner wall of the barrel due to the effect of the centrifugal force. At the same time it is desirable for the bed layer to be thin. On account of this uniform distribution of the bed in a thin layer over the entire inside wall of the barrel, it is said that a correspondingly uniform fineness is obtained at the output, but the grinding action and especially the throughput are too low in relation to the size and complexity of the apparatus. It is also disadvantageous that the balls are discharged from the mill together with the material suspension and have to be separated from the suspension outside of the mill, and then they are put back into the mill.
Apparently it is substantially for these reasons that this mill has found no practical application.
Stirring ball mills have proven practical which have stationary, vertically disposed, cylindrical barrels in which a vertical stirring shaft equipped with grinding disks is disposed; such a mill is disclosed, for example, in U.S. Pat. No. 2,855,156.
Depending on the height of these mills and the specific weight of the bed of the mixture being ground, a high hydrostatic pressure, diminishing upward from the bottom, prevails in the barrel of the mill. Accordingly, the grinding is performed under a relatively high grinding pressure that is especially high in the bottom area.
More precise studies, however, have shown that 95% of the material being ground is at the final degree of fineness at 1/4 to 1/3 of the height of the mill (measured from the inlet down); the upper 2/3 to 3/4 of the height of the mill therefore contributes hardly at all to the grinding action aside from the fact that this main part of the height of the mill is necessary in order to build up the hydrostatic pressure in the mainly effective bottom part of the height.
This relatively poor utilization of the capacity of the barrel is, of course, very unsatisfactory. Along with this is the fact that, when the mill is shut down, the bed of material separates so that the heaviest and coarsest parts sink downward. To restart the mill, therefore, a very great amount of power is needed, i.e., the drive system must be designed for this great initial power, which afterward is not utilized during normal operation.
Furthermore, on account of the compactness if the bed, these mills tend to clog from heat, so that the output of the mill is limited.
In European Patent B1 0 214 145 a dispersion method and stirring ball mill with a stationary barrel has been disclosed, in which the rotors revolve at such a high speed that, due to the centrifugal force, the balls form a rotating charge which also lies against the inside wall of the barrel, while in the center of this charge of balls a substantially free space develops. The barrel of the mill has a plurality of inlets distributed over its axial length, through which the material to be ground is distributed over the axial length and the charge of balls flows radially from the outside in against the centrifugal action such that, with respect to the balls, a centrifugal fluid bed develops and the ground material is carried out from the ball-free space through a ball separating system. The material being dispersed therefore fills the entire interior space of the grinding barrel.
The particles of the material being ground pass on a radial (or spiral) path from the inlet to the sifter disposed in the center of the barrel. This essentially radial path is too short for a sufficient, uniform comminution, so that the material has to be passed several times through the mill.
Furthermore, the centrifugal fluid bed for which this proposal strives is doubtful insofar as the radially outwardly directed centrifugal force would counteract the inwardly directed force of the flow of the particles. The bed is, so to speak, loosened up, and no grinding pressure is obtained that would correspond to the centrifugal force being exerted.
On account of the thickness of the bed, this mill tends to clog due to heat, and this again limits the throughput of the mill.
Also, a relatively high circumferential velocity and thus a relative velocity between the stirrers and the housing is necessary, entailing high attrition and wear on balls, stirrers and walls.